Partial discharges may occur along the cables of electric power transmission and/or distribution systems when localized imperfections develop in the cable insulation. For instance, a cavity in the cable insulation may well cause partial discharges under normal operating conditions or under test conditions with the cable energized at higher than rated voltage.
When a partial discharge ("PD") occurs, high frequency current and voltage pulses emanate from the site of the discharge. This is a symptom of the presence of insulation defects which must be located and assessed. A decision can then be made as to whether the cable must be repaired or replaced. In time, unattended defects may significantly deteriorate due to a combination of factors such as thermal cycling, mechanical fatigue, embrittlement, and moisture ingression. This may lead to a high concentration of electrical stress at the particular location and ultimately voltage breakdown through the insulation. Such electrical fault is normally safely cleared by properly designed protective devices. It is conceivable that some damage may still occur. Furthermore, service is interrupted for the duration of emergency repairs. The adverse economic consequences resulting from a fault may therefore be significant.
It is far more cost effective to detect partial discharging activity and determine its location soon after such conditions arise. Further diagnosis and scheduled preventive maintenance may then proceed.
Prior efforts have been made at automatic detection of fault sites along transmission lines and of fault and partial discharge sites along power cables. A majority of these efforts calculate fault (or partial discharge) location based on the propagation time of a transient fault (or partial discharge) signal.
For example, U.S. Pat. No. 4,500,834 issued to Ko et al. uses the method of reflectometry to locate ground faults or line-to-line faults. Unfortunately, conventional reflectometry may not be suitable for locating partial discharge sites due to the presence of significant levels of electrical noise and/or propagation losses. Since reflected pulses become weaker and distorted for techniques using multiple reflections from the terminals under partial discharge conditions, reflectometry is inappropriate for power cables which may extend for several miles.
Partial discharge ("PD") signals are usually very weak compared to noise. Location of PD sites using reflectometry (also known as time domain reflection or TDR methods) requires multiple reflections from the ends of the cable. As a result, the low level PD signals having to propagate over substantial distances, are further attenuated becoming difficult or impossible to measure in practice.
This limitation of reflectometry or TDR methods, is a particularly serious disadvantage when such methods are applied to lossy cables (e.g., oil paper insulated, butyl insulated, etc., and cables with resistive or no shields). Therefore, attenuation of PD signals would make conventional PD detection and location methods using TDR ineffective for lossy cables as well as, for similar reasons, for long runs of low loss cables.
The PD signal attenuation problem can, in principle, be alleviated if a second low loss cable is provided to return PD signals from the far end of the cable to the near end; this is in itself of obvious disadvantage and is impractical in most real cable installations.
Other patents, such as U.S. Pat. No. 2,717,992 to Weintraub, disclose fault location without reflectometry. These references generally show two sensors spaced along the distribution cable. Both sensors detect a propagating fault signal from a fault occurring somewhere in between, and the times of detection are centrally processed to indicate the distance of the fault from the sensors. Although the limitations of reflectometry are avoided, a separate communication channel is required from each sensor to allow centralized processing of the data; and this is a costly proposition.
An alternative arrangement can be found in U.S. Pat. No. 3,609,533 issued to Pardis et al. This reference also discloses two spaced sensors for locating faults occurring therebetween on a transmission line. However, a pulse generator 28 is provided at one sensor. When a fault signal is detected, the pulse generator is triggered to impart a timing pulse back onto the transmission line. The pulse backtracks to the second sensor, and the fault location can be determined from the time interval between the fault and timing pulses received at the second sensor. By utilizing the transmission line itself, this arrangement eliminates the need for separate communication channels. However, the Pardis et al. arrangement is designed to detect break-down wavefronts emanating from faults such as caused by lightning strikes. These wavefronts have magnitudes in the 10-100 kV range or higher. The device is ill-suited for use in detecting partial discharges, which are extremely high-frequency transient pulses (faster than propagating wavefronts by an order of magnitude), and which attain a diminutive amplitude in the millivolt range.
Pardis makes no reference to the unique problems that are encountered in partial discharge measurements. This Pardis system does not address the difficulties associated with the detection of much faster signals of diminutive magnitude, propagating in lossy mediums and measured in the presence of background noise. Pardis' system as conceived could not be used for PD detection, nor could its adaptation to that purpose be obvious to someone skilled in the art. Another difference from the present invention is in the way that Pardis measures the difference in time of arrival at the measuring station, between the first-to-arrive fault signal and the second-to-arrive injected pulse. In Pardis case, the time of arrival of the first pulse is extended by a fixed long delay; so that to the time difference measuring device, this first pulse appears as if it was the second-to-arrive at the measuring station.
The delay added has to be subtracted to arrive at the actual time differential from which the location of the fault is calculated. This way of measurement is less accurate than the present invention's due to the increased measurement and computational error associated with the offset created by the fixed delay. Finally, Pardis emphasizes the fact that his invention relates to a system, that is, to a particular way of implementing a fault location scheme.
In summary, there would be clear commercial advantage in a low cost method and apparatus for specifically locating the site(s) of partial discharges.